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Published March 24, 2011
Precipitation Verification(April 2011–September 2011)
Data Source(s): Forecast Evaluation Tool
For a thorough description of the interpretation of these maps, see the feature article, “Evaluating forecasts with the RPSS,” in the April 2009 issue of the Southwest Climate Outlook.
Comparisons of observed precipitation for April–June to forecasts issued in March for the same period suggest that forecasts have not been more accurate than equal chances in most of Arizona and New Mexico (Figure 15a). Only northwest Arizona has had increased forecast skill—a measure of the accuracy of the forecast, but the value of the RPSS is very low. For the May–July period, forecasts also have not been more accurate than equal chances; most RPSS values are close to zero (Figure 15b). For the three-month lead time, which covers part of the monsoon season, forecasts have about as good as an equal chance forecast (Figure 15c). This implies that the current forecasts for this period do not offer predictive skill. For the four-month lead time, which spans the entire monsoon period, forecasts have been substantially less accurate than equal chances in New Mexico and similar to equal chances in Arizona (Figure 15d). Regions with bluish hues suggest that the NOAA–Climate Prediction Center (CPC) forecasts have historically been more accurate than equal chances. However, caution is advised to users of the NOAA–CPC seasonal outlooks for regions where the verification maps display reddish hues.
These maps evaluate the historical performance of the one- to four-month long-lead forecasts made by NOAA’s Climate Prediction Center (CPC). The maps convey the historical accuracy of the CPC forecasts in relation to the reference forecast, which assigns a 33 percent chance to the three CPC categories, “above,” “below,” and “neutral.” These categories indicate whether conditions are predicted to be similar to the wettest, driest, or normal precipitation for 1971 to 2000. The maps are generated from the Forecast Evaluation Tool, which was developed by The University of Arizona in partnership with NOAA, NASA, NSF, and the University of California-Irvine.
The maps display the Ranked Probability Skill Score (RPSS). The more the forecasts and actual weather match, the bluer the color. A bluish or reddish RPSS indicates the forecast is more accurate or less accurate, respectively, than assigning a 33 percent chance to each of the three CPC categories.
The RPSS is calculated by comparing all the forecasts made since December 1994 for particular seasons and specified lead times to the actual weather of the season.
For more information on the Forecast Evaluation Tool, visit:
For a CLIMAS publication that explains how to use the Forecast Evaluation Tool, visit:
Southwest Climate Outlook Staff
- Michael Crimmins, UA Extension Specialist
- Stephanie Doster, Institute of the Environment Editor
- Dan Ferguson, CLIMAS Program Director
- Gregg Garfin, Founding Editor, Institute of the Environment
- Zack Guido, CLIMAS Associate Staff Scientist
- Gigi Owen, CLIMAS Assistant Staff Scientist
- Nancy J. Selover, Arizona State Climatologist
- Jessica Swetish, CLIMAS Publications Assistant
Please direct your Southwest Climate Outlook comments and suggestions to Zack Guido.
The CLIMAS Web site contains official and non-official forecasts, as well as other information. While we make every effort to verify this information, please understand that we do not warrant the accuracy of any of these materials.... Read full disclaimer